Kar Kaddy hitch coupler trouble

[Indian Creek]

I have a Kar Kaddy 3 tow dolly with surge brakes. Getting hooked up for a trip, I found that after a year and a half of use, the hitch coupler would not drop onto the hitch ball. I removed the hitch coupler assembly, inspected inside and found everthing was in good shape but the ball clamp would not go past the top half of the ball because the plunger and return spring were crowding the ball clamp, not letting it past the ball. I also found that by shimming the plunger assembly, it would not crowd the ball clamp and the coupler would clear the hitch ball and would lock properly. When finally hooked up, there were no problems, but the next time I hooked up, it was the same hassel.
Question: Does anyone know why this is happening? Have I lost a part in the assembly? Is the plunger return spring worn out already? There does not seem to be anything missing by looking at the Demco exploded view of the coupler assy. Any help would be appreciated. Thanks,
Dave

[AZgl1500]

Not at all familiar with that product, but are you using a Moly 60 paste grease on the ball? A lot of friction load there, if the ball/socket is dry, it will cause problems similar to what you are describing.